New study finds possibility of drastic and rapid climate shift in simulations with land-only planets

15.06.2018
Science
Climate
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Land

Sirisha Kalidindi and her colleagues Dr Christian H. Reick, Dr Thomas Raddatz and Prof Martin Claussen from the department "The Land in the Earth System" at the Max Planck Institute for Meteorology (MPI-M) have found a new type of instability leading to a fast planetary scale climate shift. Their findings have recently been published and highlighted in the journal Earth System Dynamics.

When studying a land-only planet called "Terra Planet" as one of the first applications of the new Earth System Model of the MPI-M (ICON-ESM), the scientists were rather surprised to find a new type of planetary scale climate instability resulting in a global cooling of about 35 K within a few years of simulation time. So far it was known that a planet like Earth may undergo a drastic climate shift either towards a very hot moist greenhouse state by evaporation of all water bodies, or to a very cold snowball Earth state by forming worldwide ice and snow covers. In contrast the new type of instability from a hot to a cold planet arises by a complex re-organization of the tropical water cycle.

The abrupt cooling is about six times the temperature difference between the present day and the last ice age. This drastic climate change is closely related to the presence of two different climate states: the Terra Planet may be found either in a state with the tropics being totally dry and very hot, or a state with drastically cooler tropics and intense precipitation (Figs. 1 and 2).

Fig. 1: Temperature and precipitation on the Terra Planet in the hot (left) and the cold (right) state. The hot state is associated with very warm temperatures and precipitation is confined to the extra-tropics. In contrast, in the cold state temperature is below 0°C almost everywhere except in the tropics, where it is around 10°C and precipitation is dominant in the tropics similar to present-day Earth.

Despite the hot state being dry in the tropics, there is also rain, but in the form of "virga rain", where the water droplets evaporate in this hot atmosphere before they reach ground. Upon decreasing solar irradiation, the height at which this evaporation happens is lowered, until it reaches the ground. This is the point where the hot state undergoes a sequence of catastrophically self-enforced transformations until the planet settles in the cold state.

Fig. 2: Global surface temperature of the Terra Planet as function of soil albedo. In the simulations instead of solar irradiation soil reflectivity (albedo) was changed, i.e. high solar irradiation corresponds to low albedo and vice versa. At low albedo (high solar irradiation), the Terra planet shows two alternative climate states, a hot one (red) and a cold one (blue). When increasing albedo (decreasing irradiation), at a certain point the hot state undergoes a catastrophic transition to the cold state (dashed line), and further on only the cold state exists. Starting from the cold state while lowering albedo (increasing solar irradiation) the planet stays in the cold state even beyond the point where the hot state broke down.

Hydrologically, the cold state is characterized by a reversed water cycle in the tropics: While in the hot state all water evaporated from the ground is exported towards the poles, in the cold state a significant fraction of this water is re-circulated into the tropics feeding the intense rain there, like on present day Earth. This type of catastrophic behaviour is possible by assuming an effective overland recycling of water from the extra-tropics to the tropics, which could happen via rivers or subsurface flows in the soils.

In their publication the scientists also speculate that Terra Planets may have a wider habitable zone than planets without effective water recycling. The habitable zone is the range of planetary distances from the sun where liquid water (as a pre-condition for life) may exist permanently. Since at the same solar irradiation the global temperature of a Terra Planet without effective overland water recycling should be found in-between the hot and the cool states of the planet with recycling, life might be possible even closer to the sun in the cool state and farther away from the sun in the hot state, so that the habitable zone should be wider.